This is a renewal application of an RO1 grant to study tolerance in peripheral B cells. Several lines of evidence, including prior work on this grant, have shown that peripheral B cell tolerance occurs, often by deletion. This tolerance is a barrier to the development of autoimmune disease. However, B cells in the periphery must also be capable of responding to foreign antigens. B lymphocytes are regulated by many signaling pathways that ensure appropriate development, activation and immune tolerance. In this proposal we focus on three pathways that regulate the development of B cell subsets in the peripheral immune system, peripheral B cell tolerance to tissue specific self-antigens, and the T-independent antibody response. Mutations affecting signaling pathways for toll-like receptors, cell surface inhibitory receptors and receptors for the TNF family cytokine BAFF will be used to probe their roles in B cell autonomous biological responses. The first Aim assesses B cell development, B cell tolerance and TI-2 responses in mice deficient in all Tlr signaling. In Aim 2, the effects on peripheral B cell tolerance and the TI-2 response of suppressing or eliminating SHP-1 in B cells will be assessed. Experiments in Aim 3 test the prediction that TACI-deficient B cells have a specific peripheral tolerance defect owing to dysregulated BAFF signaling and attempt to define the B cell autonomous role of TACI in the TI-2 response. The long term goals of these studies are to understand how the self/non-self discrimination is made, what goes wrong in the development of autoimmunity, and to identify ways that these mechanisms may be manipulated to ameliorate or prevent disease.